Mount Tamalpais

The June 16 challenge -- which 21 people got right -- was the Gravity Car Barn (part of the old Mount Tamalpais & Muir Woods Railway) near the East Peak (and the fire lookout at the top of the East Peak) of Mount Tamalpais in Marin County, Calif.

Solana solar power plant

The July 5 challenge -- which 17 people got right -- was Abengoa Solar's Solana solar power plant, a 280-megawatt parabolic trough plant currently under construction in Gila Bend, Ariz., that may end up as the world's largest such facility.

Old Trails Arch Bridge

The July 7 challenge -- which 53 people got right -- was the
Old Trails Arch Bridge, as well as other pipeline bridges that span the Colorado River (and the border between Arizona and California) between Needles, Calif., and Topock, Ariz.

Hearst Castle one-room schoolhouse

The July 10 challenge -- which 75 people got right -- was the old one-room schoolhouse in San Simeon, Calif., that was once used by children of Hearst Castle ranchhands. Hearst Castle is visible at the top of the hill in the background.

Fallen Star

The July 17 challenge -- which 140 people got right -- was artist Do Ho Suh's "Fallen Star," a small Rhode Island-style cottage hanging off the top of the Jacobs Engineering Building at the University of California at San Diego.

Mark Twain cabin

The August 1 challenge -- which 58 people got right -- was the Mark Twain Cabin, a replica of the cabin in which the author worked on "The Jumping Frog of Calaveras County," on Jackass Hill near Tuttletown, Calif.